Automatically controlled variable displacement piston pumps that are sensitive to pressure control and flow control are well known in the art. Pumps of this character are produced, for example, by the Fluid Power Division of Cessna. In operation of these pumps, when the drive shaft of the piston pump is rotated by some external source of power, a piston block, which is splined to a drive shaft, also turns. The piston block contains a plurality of piston assemblies which have a free swiveling shoe swaged on the ball end of the piston assembly. The shoe end of the piston rides against a smooth surface of a camplate. When the camplate is in its neutral position the piston assemblies do not travel in and out of the piston block bores. They remain in a stationary position in relation to the piston block and rotate merely as an internal part of the piston block. When the camplate is in the neutral position no oil is being drawn into the pump and no oil is being discharged from the pump. The pump is then in a zero displacement position.
When the camplate is moved to any position from neutral up to a full cam angle, such as 17.degree., the piston shoes follow the inclined surface of the camplate and reciprocate in the piston block bores. Half of the piston assemblies are being pulled out of the piston block while the remaining half of the pistons are being pushed back into the piston block. This action causes the piston pump to pump oil, the pistons being pulled from the piston block to suck oil into the piston block bores through the suction port and an intake kidney slot. As the pistons cross over top dead center the pistons push the oil out of the piston block bores and into the pressure kidney slot and on out the pressure port. Each piston assembly completes the cycle each revolution of the pump shaft causing a continuous flow of oil from the pump. The greater the cam angle, the greater the piston stroke, the greater the piston stroke, the more oil is pulled into the pump and discharged out the pressure port.
Pressure compensator control apparatus and flow compensator control apparatus that provide system efficiency by matching both pressure and flow to load requirements are conventionally used in conjunction with variable displacement piston pumps of the foregoing character. When control apparatus of this type is used and when pressurized oil is required in the system, the flow of oil is regulated by the differential in pressure. When the pump is required to provide more flow to the system, the pressure in the pump pressure passage will drop slightly. This allows the oil from a camplate control piston of the pump to drain and the pump then increases its volume of discharge. When the flow is satisfied the pump camplate will stabilize in a position to provide the flow required. The flow rate of oil in the system is dictated by an orifice in a control valve. This orifice can be spool travel, a flow control valve or passage size. Whatever method is used, it is sized to provide the desired flow at a constant pressure drop across the regulating orifice. The pressure in the system is fed from the load down stream of the orifice back to the sensor port in the pump compensator. Since the strength of the flow compensator spring is set at a fixed amount, such as 200 P.S.I., the spring pressure plus the load pressure fed back to the compensator will require the pump to pump the flow rate of oil dictated by the size of the orifice at a constant pressure (200 P.S.I.) above the actual load pressure. The pump will then provide only the flow rate required at constant pressure (200 P.S.I.) above actual load pressure. The pump will thus automatically adjust to the varying pressure and flow demands of the system.
The known pressure and flow compensator control apparatus described above contributes significantly to the saving of energy in the system. A still further need in systems of this type, in addition to the savings associated with pressure and flow controls, is to provide horsepower limiting controls that limit the horsepower delivered by the pump to the amount of the pump driving means that is made available, which can be accomplished by reducing the pump output flow to meet the limitation of the driving means.
It is known to provide pump assemblies that have pressure and power controls, but in these known assemblies the power limiting controls are constructed within the pumps, resulting in especially built units that are costly, and the power limiting controls cannot be adapted for use with standardized pump assemblies having known pressure and flow compensator control apparatus. In the known power limiting controls, it is the conventional practice to employ pilot fluid in the pump assemblies for flow through a variable orifice for regulating the pump operation which results in a control system of considerabove complexity being built within the pump, and one which cannot be readily used with standard pump assemblies having known pressure and flow compensator control apparatus.